PSYCHOACTIVE SUBSTANCE USE DISORDERS:

Drug: It is derived from a French word 'drogue'. A medical substance used in the treatment of disease. (Taber's  Dictionary). 

'Drug dependencelsubstance  dependence: A state psychic, sometimes also physical, resulting from taking a drug characterized by behavioural and other responses that always include a compulsion to take a drug on a continuous or periodic basis in order to experience  its psychic effects and sometimes to avoid the discomfort of its absence. Tolerance may or may not be present. 

Physical dependence: Physical dependence occurs when  the drug user's  body becomes so accustomed to a particular drug that slhe  can fundtion normally only if the drug is present  in  the body chemistry. 

Psychic Dependence:  Psychic or psychological  dependence occurs when the drug is central to a person's thought, emotions and activities. It is difficult for the person to stop thinking of  the drug which causes intense craving for it and its effect.  . 

Tolerance: It is 'an  adaptive state characterized by a diminished  response to the same quantity of a drug or by the fact that a larger dose is required to produce the same degree of pharmacodynamic effect'. (WHO Bulletin, 1965) 

Withdrawal:  It is an organic mental disorder following  cessation (stopping)  or reduction in the intake of a substance such as alcohol, an opioid, amphetamines, tobacco or sedatives that have previously been used regularly to induce intoxication.